Such an NMR construction is by way of example known in the art from DE OS 31 33 873.
An important component of this type of NMR system, which is normally used for nuclear spin tomography, is a system of three gradient coils which, independent of another, are fed with currents of different strengths. These coils serve the purpose of overlapping constant field gradients of adjustable strengths upon the homogeneous magnetic field B.sub.0 of the main field coil, whereby the direction of one of these gradients (dB.sub.z /dz) is, as a rule, parallel to the direction of the homogeneous main field B.sub.0z, that is to say, to the axis of the main field magnet (z gradient=axial gradient) and the directions of the two other gradients (dB.sub.z /dx, dB.sub.z /dy) run orthogonal thereto and with respect to each other transverse to the direction of the main field (x and y gradients=transverse gradients). The spatial region in which the magnetic field of these gradient coils runs approximately linearly can be utilized for spatially resolved NMR methods (imaging, position selective spectroscopy) in so far as this region is not further limited through inhomogeneities of the main field. The gradient coils are, as a rule, attached to a cylindrical support pipe which surrounds the patient.
Due to the geometric configuration of conventional gradient coils, the support pipe has an axial extent to both sides of the center of the linear region which assumes a value of 0.6 to 1.5 times the diameter of the support pipe. With the typical value of 0.7 m for the diameter of the support pipe, this is 0.42 m to 1.05 m. The patient is therefore surrounded by a relatively long narrow pipe. With sensitive patients, this can easily lead to conditions of claustrophobia. A further disadvantage of conventional gradient coils systems is due to the fact that is is not possible, in special body investigations as by way of example the examination of the head or the extremities, to utilize gradient coils in proximity to the object since the coils, due to the large axial separation from the linear region up to the ends of the support pipe, must surround the shoulders of the patient, that is to say must have a diameter of at least 0.5 m. Gradient coils in proximity to the object with small diameters had namely the advantage of a substantially reduced inductivity for a given gradient strength per unit current, through which correspondingly smaller gradient rise times were made possible. This is particularly advantageous for the executability of modern NMR examination methods (echoplanar methods etc.).
The relatively large separation from the axial ends of a conventional gradient coil system to the center of the linear investigation region is largely caused by the geometry of the transverse gradient coils. Such a conventional coil configuration consists of 4 partial coils arranged symmetrically upon the surface of a cylinder of radius R. The coil configuration is largely mirror symmetric with respect to the z=0 plane as well as to the y=0 plane in the case of y gradient coils or to the x=0 plane in the event of x gradient coils respectively. Furthermore, both partial coils possess in the region of z&lt;0, considering the flux direction of the electric current, a common winding direction, for example, clockwise with respect to the axis given by the gradient direction, with the two partial coils in the region of z&gt;0 exhibiting the opposite winding direction.
Due to the mirror symmetry of such coils a magnetic field is then produced independent of the detailed guiding of the windings which has the same symmetry as the desired transverse gradient (x or y gradient), that is to say, is an even function with respect to the z=0 plane and an odd function with respect to the y=0 plane. The detailed guiding of the windings is thereby so chosen that the deviation of the z component of the magnetic field from the desired function B.sub.z =ay is small over as large a region as possible.
The undesired large axial extent of such coils is obviously caused by the "return current section" of the windings which, in order to produce a sufficiently large volume with approximately linear field dependence (by way of example with a relative deviation from ideal linear dependence of .+-.5% in a spherical volume with a diameter of 50% of the cylinder diameter d characterizing the gradient coil), must lie in the region of z=.+-.0.6 d. In view of the compromising of the linearity by return portions of the winding of relatively small distance z from the central plane, it is obvious that these portions, in the region of their axial positions, produce a gradient which is oppositely directed to the field gradient produced by the winding sections in the region of the mid-plane z=0. A large axial extent of the linear investigation region is thereby only possible with gradients whose return sections exhibit a large separation from the central plane z=0.
Towards this end it is therefore necessary that a patient, of which by way of example a tomogram of the inner skull is to be taken, be inserted head first over a long axial stretch along the z-axis into a long narrow pipe so that the head of the patient comes to rest in the measurement volume in the middle z=0 plane. In the event that the diameter d of the gradient coil, by way of example for the purpose of realizing high gradient strengths or small inductivity, is less than the shoulder width of the patient, the patient upon insertion into the magnet, would be stuck at the shoulders and the head would not be able to be brought at all into the axially distant measurement volume.
European laid open publication 0399789A2 discloses the use of a asymmetric transverse gradient coil in conjunction with fringe field imaging. In the fringe field imaging procedure, the axial gradient is generated by the inhomogeneous magnetic field at the edge of the magnet. Although these gradients can be extremely strong, their linearity is compromised. In conjunction with this system, symmetric axial gradient coils, to supplement the axial gradient of the fringe field, are disclosed. There is no mention of asymmetric shielding coils in conjunction with the system.
European laid open publication 0108421A2 discloses the use of an asymmetric axial gradient coil in conjunction with transverse coil systems for the purpose of displacing the center of the gradient region for imaging. Asymmetric transverse coils are not proposed nor are they relevant to this particular publication. The use of active shielding is discussed in Europan patent laid open publication 0433002A2 and a possible geometric configuration for asymmetric transverse gradient coils is suggested in the abstracts to the Society of Magnetic Resonance in Medicine Meeting, Aug. 1991, page 711.
In view of the above, it is therefore the purpose of the present invention to present an improved NMR apparatus that, on the one hand, compensates for patient claustrophobia while, on the other hand, allowing head examinations using an apparatus whose inner diameter clearance can be, for example, larger than the diameter of a human head, but smaller than the average shoulder width of a person.